The present invention relates to a method for controlling at least one prime mover adapted to drive at least one ground engaging element of a working machine and the working machine.
The invention is applicable on working machines within the field of industrial construction machines, in particular wheel loaders. Thus, the invention will be described with respect to wheel loaders. However, the invention is by no means limited to a particular working machine. On the contrary, the invention may be used in a plurality of heavy working machines, e.g. articulated haulers, trucks, bulldozers and excavators.
A wheel loader is usually provided with an internal combustion engine, a traction system for operating the loader on the ground and a hydraulic system for operating an attachment. The machine normally comprises a front body section pivoting with a rear body section and a loading unit with a linkage and an attachment in the form of a bucket (or fork or log grapple) fitted on the linkage. The hydraulic system operates the linkage and the attachment. By operating the hydraulic system, the loader can load the bucket with for instance gravel. The system may be electronically and/or hydraulically controlled. Blends between an all-mechanic-hydraulic control and all-electronic control are common and known to persons skilled in the art.
The traction system operates the working machine on the ground. The power from the engine is fed to a torque converter, which further feeds the power via transmission axles to ground engaging elements, in form of wheels. Since the wheels acts on the ground through penetration and traction, there will be a traction force coupling between the engine and the ground.
The internal combustion engine normally used in wheel loaders is controlled by an engine control unit (E-ECU), which controls the engine on the basis of an operator control input. The operator control input is a voltage or current value created when the operator pushes the accelerator pedal. The operator control input forms a desired value for the engine speed (in % or an interval such as 0 to 1). The E-ECU could be a separate unit or functionality within a vehicle ECU that controls the complete operation of the loader. The vehicle ECU makes sure to control the engine on the basis of the total power need of the loader.
Today the internal combustion engine in the wheel loader is variable-speed controlled. This means that the operation signal indicates a certain desired value for the speed. The E-ECU internally calculates a required value for the torque to make sure the engine holds the speed. When the load on the wheel loader changes, the E-ECU controls the actual engine speed by changing the required value for the torque.
In future wheel loaders it might be advantageous to use series hybrid systems where the internal combustion engine drives a generator to charge a battery. A plurality of electric motors, powered by the battery, then provides power to the hydraulic system and the traction system. The traction system of such loaders has no torque converter.
In a conventional wheel loader, the torque converter's characteristics and its elasticity gives the operator an indication of traction force at low vehicle speeds and constant traction resistance, e.g. when filling the bucket. In a hybrid system without torque converter, it is desirable to still give the operator such an indication of traction force. Otherwise, in a variable speed-controlled machine the operator's command of the gas pedal will result in that the wheels skids, with a negative experience for the operator. Moreover, since the wheels skid, they will wear out rapidly, which is negative for the operating economy of the vehicle.
The same problem will apply also to wheel loaders having a parallel hybrid system and a traction system with no torque converter. In such a system, the internal combustion engine and the electric motor both power the hydraulic system and the traction system.
It will also apply to conventional non-hybrid loader, powered by an internal combustion, and having a traction system with no torque converter.
It is desirable to solve the above mentioned problem by improving the control of a prime mover in a working machine without a torque converter.
An aspect of the present invention relates to a method for controlling a prime mover adapted to drive at least one ground engaging element of a working machine. The method comprises the steps of: (1) receiving an operator control input indicative of the control of the prime mover and (2) determining at least one operation signal in response to the operator control input, which operation signal is sent for controlling the prime mover.
What particularly characterizes the method is the steps of: (3) receiving a operating state input indicative of an operating state of the machine and (4) selecting a control mode from at least one speed control mode in which the determined operational signal comprises a desired speed of the prime mover and at least one torque control mode in which the determined operational signal comprises a desired torque of the prime mover in response to the operating state input.
With the present method, the operability of working machines, e.g. a wheel loader, that does not have a torque converter, is improved. By controlling the prime mover either by speed or by torque, depending on the situation, the machine can be adapted for different operating states and thereby the operability of the machine is improved.
The control mode may shift from the speed control mode to the torque control mode when the machine switches to a kick-down operating state and shift from the torque control mode to the speed control mode when the machine switches to a reverse gear operating state. The method may furthermore select between one of a plurality of speed control modes and a plurality of torque control modes. Each control mode may comprise at least one control map for determining the operation signal. A shift between control modes on the basis of the kick-down or reverse means that the machine can adapt to the operating conditions on the basis of these shifts to improve the operability. Using control maps improves this operability.
In the torque control mode the actual value for the speed of the prime mover at least depends on the load acting on the machine, which improves the feedback to the operator.
The prime mover may be an internal combustion engine or an electric motor, where the electric motor may be part of a series hybrid system. As an alternative, the prime mover is a parallel hybrid system with an internal combustion engine and an electric motor connected in parallel. The selected control mode may therefore control the internal combustion engine and/the electric motor. It is often that prime movers comprises an electric motor and is not connected to a torque converter. The use of this method in such prime movers significantly improves the operability.
The operating state input may be determined by the operator's actuation of a control device for selection of different operating states, wherein this control device could consist of or comprise a kick-down button. As an alternative the operating state input is determined automatically based on at least one detected operating condition. An automatic determination can eliminate the need for an operator input like a kick-down button or gear lever while a manually actuated control device is preferred by some operators.
Another aspect of the present invention relates to an engine control unit or a hybrid control unit being adapted to perform any of the method steps according to the method noted above.